From German "Auslegeschrift" No. 1,281,049 it is known to construct a nuclear reactor, operating according to the counter current principle, in such a way that a first feed water flow is fed in directly at the lower end of the reactor core whereas a second feed water flow is first subjected to a certain pre-heating by allowing it to flow downwards through the core but without making direct contact with the fuel rods, before it is connected with the first feed water flow. The reactor disclosed in the above cited publication is a pressurized water reactor of a special design, intended to be fed with cooling water of supercritical pressure, with the flow of coolant which is fed there in being equal to the flow of coolant which is leaving the reactor vessel.